For those of you who have been following along, the tl;dr answer is:
() -> Void
Everyone else: The Swift Programming Language ebook refers exclusively to () -> (). APIs all use () -> Void.
I much prefer the Void typealias because it better matches return conventions in the Objective-C world.
And now, word of Apple at devforums: “FWIW, we’ve recently decided to standardize on () -> Void (generally, () for parameters and Void for return types) across all of our documentation.”